The transmission of a signal in a multi-path environment results in a plurality of delayed versions of the signal being received by receiving circuitry. Such multi-path environments are for example the result of obstacles in the path between the transmitter and the receiver, such as buildings, features of the landscape, etc.
Circuits for receiving signals in multi-path environments generally comprise a channel equalizer for summing all of the received signals and recovering the original signal. Such channel equalizers generally comprise a filter having many taps. Each tap multiplies a delayed version of the input signal by a certain coefficient, and the outputs from all of the taps are added together to generate an output signal. The coefficients of each tap are generally calibrated for a particular environment at the start of a transmission, and may require recalibration at regular intervals, particularly in the case of dynamic transmission channels.
FIG. 1 illustrates an example of an adaptive filter 100 of a channel equalizer for receiving a signal r(t) received from a multi-path environment. Filter 100 comprises an input for receiving signal r(t), which is, for example, received by an aerial, and a series of delay elements 102 to 107, which each delay signal r(t) by a determined amount. Signal r(t) is coupled to a first multiplier 108, which multiplies this signal by a coefficient C0. The outputs of the delay elements 102 to 107 are coupled to respective multipliers 109 to 115, which multiply these signals by respective coefficients C1 to Cn. The outputs of multipliers 108 to 115 are added together by an adder 116 to generate an equalized signal y(t). Filter 100 is an adaptive filter as the coefficients of the taps are updated based on a feedback signal, allowing the frequency response of the filter to be updated.
In general, to deal with harsh multi-path environments, the filter of FIG. 1 may include as many as several thousand taps, and therefore several thousand coefficients that need to be adapted. During an adaptation phase, the values of the coefficients C0 to Cn are calibrated for the particular environment. Generally the coefficients are updated in response to each data signal that is received. However, due to the large number of taps, this is very slow and/or very demanding on processing power. It is a particular problem in the case of dynamic channels in which the adaptation speed may well be too slow to enable the tracking of the channels, resulting in picture loss.
Some methods have been proposed for achieving better adaptation speed. However, these generally involve matrix inversions, which require a lot of processing power and are very demanding in terms of hardware resources, making the channel equalizer bulky and expensive.
There is a need for circuitry and method for updating the filter coefficients that is both fast and efficient in terms of its hardware requirements.